The present invention relates to a vehicle power assisted steering system, more particularly to such a system of the type known as speed proportional whereby the power assistance which is provided decreases as vehicle speed increases and conversely. Systems of the aforementioned type are known as comprising an hydraulic pump having a constant volume output which communicates with an open centre power assistance valve that is adjustable from a neutral condition in response to a steering input torque during a steering maneuver to control fluid flow from the pump to opposed chambers of a ram which provides power assistance to assist the maneuver. The power assistance valve has valve members relative displacement (typically rotational displacement) between which from the neutral condition and in response to a steering input torque directs hydraulic fluid to actuate the ram for power assistance. In the neutral condition of the open center; power assistance valve, the valve members provide inter-communication between the pump, the chambers of the ram and the reservoir and these valve members are biased to the neutral condition by a biasing means. In the case of relatively rotational valve members the biasing means is usually either or both of a torsion rod or C-spring component which reacts between the valve members, for example in the manner disclosed in our British Patent Specification No. 2,165,502.
For the purpose of providing speed proportional power assisted steering with the system as discussed above, it has hitherto been proposed, for example in our British Patent Specification No. 1,293,192, to provide a bypass valve which communicates between the pump outlet and a fluid reservoir and which is responsive to vehicle speed so that the by-pass valve increasingly opens communication between the pump outlet and the reservoir as vehicle speed increases and conversely. By this arrangement the hydraulic fluid pressure differential between the pump outlet and the reservoir across the open center power assistance valve decreases as vehicle speed increases; therefore as vehicle speed increases there is an apparent decrease in fluid pressure available to actuate the power assistance ram and thereby the "feel" of the steering becomes heavier during a maneuver as the steering approaches what may be regarded as wholly manual steering characteristics. With such a speed proportional system, upon displacement of the open center power assistance valve from its neutral condition in response to a steering input torque, the opposed chambers of the ram are closed from communication with each other while the expanding chamber maintains its communication with the pump outlet and the contracting chamber maintains its communication with the reservoir in the known steering systems. It is recognised that this can have serious adverse consequences during normal steering of a vehicle at high speed where the by-pass valve is adjusted to a condition in which very little or no hydraulic pressure is available across the power assistance valve to assist in displacement of the ram. During this latter condition it is possible in some circumstances to allow too much fluid to be by-passed. This can occur if a severe and rapid steering maneuver is to be carried out at high vehicle speeds. This causes the system pressure to rise abnormally because the by-pass valve is fully open at these high vehicle speeds and the demanded pressure will cause large oil flows to be by-passed. The effect of this is that all power assistance is suddenly eliminated and this condition is not acceptable. It is an object of the present invention to provide a vehicle power assisted steering system of the speed proportional type and which alleviates the problem discussed above.